The present invention, in some embodiments thereof, relates to manufacture of loose tubes for fiberoptic cables, post extrusion shrinkage, and more particularly but not exclusively, to a way of mitigating or overcoming the effects of post extrusion shrinkage (PES) in loose tube fiber optic cables.
Loose tube fiber optic cables are commonly employed in long-haul, metro-core, access, distribution, indoor or outdoor, premises and even in submarine applications. One parameter of the loose tube design is excess fiber length. Excess fiber length can be defined as the additional physical fiber length as compared to the linear physical length of the loose tube in which the fibers are contained.
Excess fiber length in a loose tube is a parameter primarily to be controlled during loose tube manufacturing, commonly through the use of an interim capstan, and if executed, also during the stranding of said loose tubes. If excess fiber length is above a certain threshold, it will harm the optical performance of the cable. Likewise, if the excess fiber length is too low.
Loose tubes are considered for multi-loose tube cables, although the issue applies mutatis mutandis to Central tube (Mono-tube or Uni-tube) cables. Multi-loose tube cables are stranded around a central strength member. Color coded loose tubes are fed to a stranding unit. Feeding is via dancer and pulley units. Controlled tension is applied on the loose tubes to keep them taut while stranding, and may cause elongation of the loose tubes. The effect of tension i.e., elongation or strain on loose tube can be calculated using the following formula:Elongation in %=T/Y×A×100    where T is the tension applied in kg, Y is the Young's modulus of loose tube plastic material in kg/mm2 and A is the cross sectional area of loose tube in mm2.
A typical tension to be applied to the loose tube is generally around 250 grams. This value is required to be changed slightly based on the actual condition of the loose tube. If the tubes show looseness, the tension may be increased, provided we are confident that the excess fiber length in the tubes can withstand the new load, and the above formula may be used. The strain due to the tension should be equal to or less than the excess fiber length. In other words the excess fiber length in the loose tubes should nullify the effect of strain on the fiber induced due to elongation of tubes at stranding.
Fiber excess length present in the loose tube which is used in multi-loose tube fiber optic cables takes into account tube elongation caused during the stranding process as well as during cable installation.
Thus, in multi-loose tube cables, affecting excess fiber length may be achieved through the stranding process in addition to that achieved during the process of producing the loose tube itself. There is no stranding in central tube cables nor is there a central strength member around which multiple tubes are stranded. Hence, during central tube cable manufacturing the process often is required to provide more excess fiber length compared to that of the loose tube for stranded multi-loose tube cables.
The issue of post extrusion shrinkage is discussed in U.S. patent application Ser. No. 11/039,122 to Wayne Kachmar.
A fiber optic cable typically includes: a fiber or fibers; a buffer or buffers that surrounds the fiber or fibers; a strength layer that surrounds the buffer or buffers; and an outer jacket. Optical fibers function to carry optical signals. A typical optical fiber includes an inner core surrounded by a cladding that is covered by a dual layer coating. Buffers typically function to surround and protect coated optical fibers. Strength layers add mechanical strength to fiber optic cables to protect the internal optical fibers against stresses applied to the cables during installation and thereafter. Examples of strength layers include aramid yarn, steel and epoxy reinforced glass roving. Outer jackets provide protection against damage caused by crushing, abrasions, and other physical damage. Outer jackets also provide protection against chemical damage (e.g., ozone, alkali, acids) and environmental effects (UV rays from sunlight etc.).
It is well known that macro-bending of an optical fiber within a cable will negatively affect optical performance. Shrinkage of the outer jacket of a fiber optic cable can cause axial stress to be applied to the optical fiber, which causes a cyclical/periodic macro-bending of the optical fiber. One cause of jacket shrinkage is thermal contraction caused by decreases in temperature and crystallization of the molecular structure of the polymeric extrudate. Another source of shrinkage is post-extrusion shrinkage.
Shrinkage caused by thermal contraction is typically only temporary or elastic in nature. The amount of thermal expansion/contraction is dependent upon the coefficients of thermal expansion of the materials involved. In a typical fiber optic cable, the jacket has a higher coefficient of thermal expansion than the fiber. Thus, when the temperature drops due to normal environmental temperature cycling, the jacket may shrink more than the fiber causing stresses to be applied to the fiber. These stresses are typically only temporary or elastic, since the jacket will expand back to its original size when the temperature returns to ambient.
Post-extrusion shrinkage is a by-product of the extrusion process used to manufacture fiber optic cables. Generally, to make a fiber optic cable, an optical fiber is passed through an extrusion die and molten plastic material is extruded about the exterior of the fiber. As the molten plastic exits the extrusion die, it is elongated in the direction of flow and then passed through cooling baths where the elongated shape of the plastic is set. However, after the shape has been set, the plastic material continues to have “memory” of the pre-elongated shape.
Thus, if the cable is later heated, the plastic material will gravitate towards its pre-elongated shape thereby causing post-extrusion axial shrinkage of the cable jacket. As indicated above, cable jacket shrinkage can cause macro-bending of the optical fiber thereby degrading signal quality. Unlike shrinkage caused by thermal contraction, post-extrusion shrinkage of the type described above is permanent or plastic in nature.
Post-extrusion shrinkage is a significant problem in the area of optical fiber connectorizing. When a connector is mounted to the end of a fiber optic cable, a heat cure epoxy is often used to secure the connector to the jacket and strength layer. When the epoxy is heated during the cure cycle, the cable jacket is also heated thereby causing permanent post-extrusion shrinkage. Post-extrusion shrinkage can also be caused after installation by environmental temperature variations, or even after a cable is released from the winding tensions on the shipping drum, enabling the cable jacket to contract.
In general, excess fiber length is designed to be within the elastic deformation region of the cable and the cable can shrink or elongate to cover the excess length, with the amount of shrinkage or elongation varying depending on environmental temperature and cycling during the course of the life of the cable as well as additional axial tensions present. Post extrusion shrinkage however tends to be in the plastic deformation region and is permanent.
The existing art deals with post extrusion shrinkage through material constriction. In addition, the existing art deals with excess fiber length as part of the manufacturing process. The two different effects are of different orders of magnitude and occur at different timescales.
Additional background art includes U.S. Pat. Nos. 7,379,642 6,801,695 7,346,244 6,324,324 8,798,416